1. Field of the Invention
The present invention is directed to a sintered ceramic-metal composite product and a method of fabricating the same.
2. Description of the Prior Art
Sintered polycrystalline ceramic products, due to their excellent heat, wear and corrosion resistance, are currently expected to have a wide variety of applications for use as forming turbo-charger rotors of automobiles, various cutting tools and tool bits, mechanical seals, and even sporting and leisure goods. However, because of inherently strong covalent and ionic bond that permit no substantial dislocation or plastic deformation as opposed to metallic materials, the ceramics fail to alleviate stress concentration occurring at a leading end of a crack and are therefore easily fractured as a consequence of that the crack proceeds as started from a minute internal defect or a surface scratch. Thus, the ceramics exhibit poor toughness with attendant brittleness which excludes the application of the ceramics for making large parts or parts of complicated shapes and therefore restricts the application only to parts of limited dimensions and shapes.
In order to overcome the brittleness, attempts have been made to provide a ceramic matrix composite of improved toughness and strength in which minute particles or whiskers of one ceramic material are dispersed into a matrix of another ceramic material. Such ceramic matrix composite strengthening has advanced from the ceramic particle dispersion to whisker or fibre dispersion, and from a micro-order inter-granular dispersion to a nano-order intra-granular dispersion. Particularly, the ceramic matrix composite of the nano-order dispersion [hereinafter referred to simply as nano-order ceramic composite] is reported to show remarkable improvement in mechanical strength, particularly at high temperature, for example, as disclosed in Japanese patent non-examined early publication (KOKAI) No. 64-87552. The patent teaches to strengthen .alpha.-alumina matrix by dispersing minute SiC particles within the granules of the alumina matrix. Besides, it is also known that improved mechanical strength is achieved in other nano-order ceramics composites of Al.sub.2 O.sub.3 /Si.sub.3 N.sub.4 where Si.sub.3 N.sub.4 is an intra-granular phase in the matrix of Al.sub.2 O.sub.3 and MgO/SiC where SiC is an intra-granular phase in MgO matrix. As for a non-oxide ceramic matrix, a like nano-order ceramic composite of Si.sub.3 N.sub. 4 /SiC is known to exhibit improved strength, as described in the publication "Powder and Powder Metallurgy Vol. 1.36 page 243, 1989". The publication teaches to react [Si(CH).sub.3 ].sub.2 NH in an atmosphere of ammonia and hydrogen through chemical vapor deposition (CVD) technique to obtain amorphous composite powder of Si--C--N which is subsequently processed to present the corresponding ceramic composite Si.sub.3 N.sub.4 /SiC in which SiC particles are dispersed within the granules of the Si.sub.3 N.sub.4 matrix.
With regard to the toughness, the ceramic matrix composite of the micro-order dispersion [hereinafter referred to simply as micro-order ceramic composite] can exhibit an improved fracture toughness of about 10 MPam.sup.1/2 for a ceramic matrix in which ZrO.sub.2 particles or whiskers are dispersed and an even more improved fracture toughness of about 20 to 30 MPam.sup.178 for a ceramic matrix in which long fibers of SiC are dispersed. While, on the other hand, the nano-order ceramic composite is found to exhibit the fracture toughness which is only about 30 to 40% of that for the micro-order ceramic composite. Despite the poor fracture toughness, the nano-order ceramic composite has an improved mechanical strength of as much as 160 to 200% of that of the micro-order ceramic composite. Consequently, it is most desirous to further improve fracture toughness of the nano-order ceramic composite so that the nano-order ceramic composite can combine excellent toughness and fracture strength.